1. Field
One embodiment of the invention relates to a storage medium such as an optical disk, a reproducing method, and a recording method of the storage medium capable of recording and reproducing information by using a laser beam with a short wavelength such as a blue laser beam.
2. Description of the Related Art
As is well known, in recent years, with prevalence of personal computers or the like, media for storing digital data has been increasingly important. For example, currently, an information storage medium capable of digitally recording and reproducing video image information and audio information or the like for a long period of time is prevalent. In addition, an information storage medium for digital recording and reproduction is used for a mobile device such as a cellular phone.
Here, an information storage medium of this type is often utilized as being formed in the shape of a disk because the medium has a large capacity of recording information; and has random access performance which can make a search for desired recording information speedily; and moreover, the medium is small in size and light in weight, excellent in storage property and portability, and inexpensive.
In addition, as an information storage medium of such a disk shape, currently, there is mainly used a so called optical disk capable of recording and reproducing information in a contactless manner by irradiating a laser beam. This optical disk mainly conforms to a compact disk (CD) standard and a digital versatile disk (DVD) standard, and has compatibility between these standards.
There are three types of optical disks, i.e., a read-only type which is not capable of recording information such as a CD-DA (Digital Audio), a CD-ROM (Read Only Memory), a DVD-V (Video), or a DVD-ROM; a write-once type which is capable of writing information only once such as a CD-R (Recordable) or a DVD-R; and a rewritable type which is capable of writing information any number of times such as a CD-RW (Rewritable) or a DVD-RW.
Among them, as a disk capable of recording information, a write-once type optical disk using an organic dye for a recording layer is the most prevalent because of its low manufacturing cost. This is because, if an information recording capacity exceeds 700 MB (Mega Bytes), there is almost no need of erasing recorded information and rewriting new information and only one recording will suffice eventually.
In a write-once type optical disk using an organic dye for a recording layer, after a recording area (track) defined by a groove has been irradiated with a laser beam, if a resin substrate is excessively heated at a point equal to or greater than its glass transition point Tg, an organic dye film in the groove generates an opto-chemical reaction, and a negative pressure is generated. As a result, the resin substrate is deformed in the groove. A recording mark is formed by utilizing the resulting deformation.
A typical organic dye used for a CD-R in which wavelength of a laser beam for recording and reproduction is about 780 nm includes a phthalocyanine based dye such as IRGAPHOR Ultragroon MX available from Ciba Specialty Chemicals. In addition, a typical organic dye used for a DVD-R in which wavelength of a laser beam for recording and reproduction is about 650 nm includes an azo metal complex available from Mitsubishi Kagaku Media Co., Ltd.
In the meantime, in a next generation optical disk which achieves recording and reproduction with higher density and higher performance as compared with a current optical disk, a blue laser beam having a short wavelength in the order of 405 nm is used as a laser beam for recording and reproduction. However, an organic dye material capable of achieving practically sufficient recording and reproducing characteristics by using such a light beam having a short wavelength has not been developed yet.
In Japanese Patent Application KOKAI Publication No. 2002-74740, there is disclosed an optical storage medium including a recording layer formed of an organic dye compound in which a wavelength showing a maximum absorption is longer than a wavelength of a recording light beam. However, in Japanese Patent Application KOKAI Publication No. 2002-74740, for example, there is nowhere described a configuration of enhancing performance of an optical disk itself such as durability in the case where a mark recorded in an optical disk has been continuously reproduced by a blue laser beam pickup. In the case where a certain track has been continuously reproduced, there is a need for preventing signal degradation. A write-once type optical disk using a dye excellent for reproduction light stability is desired.
As described above, a storage medium using a conventional organic dye material is not sufficient in reproduction durability count.